Photosynthesis

(3) Photosynthetic Model Systems for Photochemical Energy Conversion

The objective of this work is to understand how the parameters
that lead to efficient photoinduced electron transfer in natural
photosynthetic systems contribute to the efficiency of solar energy
conversion in semiconductor-based model systems. The
reproduction of the early stages of natural photosynthesis is a
necessary step for designing and optimizing efficient artificial
photosynthetic systems. We study the role of sequential electron
transfer, and electrochemical parameters, to understand how they
affect charge separation distance and efficiency. In current work
we build upon our previous understanding of the enhanced
coupling between metal oxide surfaces and modifiers having
bidentate bihydroxyl ligands. Special binding mechanisms lead to
an enhanced rate of forward electron transfer and delayed back
electron transfer. In order to follow the correlation of the rate of
back electron transfer with the charge separation distance, we
couple the surface modifier with oligonucleotides of different
sequences. The sequence is designed to have hole trapping sites
at different distances from the particle surface. Moreover, we
seek to understand the forces that finely tune the electron transfer
in model systems, such as hydrophobic/hydrophilic interactions,
local electrostatic fields, or formation of secondary structures
obtained by self organization.

Recent Research

Surface modification of nanocrystalline metal oxide particles with
ortho-substituted hydroxylated aromatic ligands was found to
result in bidentate coordination of surface Ti atoms. We have also
found that due to this specific binding of surface modifiers, the
optical properties of small titania particles change and the onset of
the optical absorption shifts to the red, compared to unmodified
nanocrystallites. The red shift is proportional to the number of p
electrons in the ligand participating in the charge transfer complex
(for dihydroxy cyclobutene dione 0.8 eV, ascorbate 1.6 eV and
dopamine 1.85 eV). The binding was found to be exclusively
characteristic of small particle colloids in the nanocrystalline
domain where surface Ti sites are coordinatively unsaturated.
Using XANES and EXAFS, we have previously shown that the
unique binding is a consequence of adsorption-induced
reconstruction of the nanoparticle surface.

The appearance of absorption in the visible region by surface
modification provides an alternative method for light sensitization
of large band gap semiconductors. These newly developed systems
have an important feature in that charge pairs are instantaneously
separated into two phases following photoexcitation. Using
continuous wave (cw) EPR we were able to establish identities
of radical species formed by excitation (visible light, l > 500 nm)
of the charge transfer complex of 40 Å TiO₂ nanoparticles with
dopamine or ascorbic acid. Two light-induced reversible signals
attributed to oxidized donor and reduced acceptor in the continuous
light cw EPR spectrum were obtained. The first signal (g₁= 1.988,
g₂=1.961 and g_2'=1.958 DH_pp = 2.5 Gauss) is characteristic of
a radical in which the unpaired electron occupies the d-orbitals of
Ti atoms having a strong component of angular momentum. [1
stands for perpendicular; 2 stands for parallel]. No EPR
signals associated with the surface components were observed
(surface Ti centers, g₁=1.924, g₂=1.885), indicating that dopamine
(ascorbate) ligands raise the energy of the surface trapping sites.
The second EPR signal at g=2.004 was dependent on the ligands
used for surface modification. In the case of modification with
ascorbate ligands, DH_pp was 11 Gauss. A similar but broader
signal with g=2.004, DH_pp = 16 Gauss was obtained when
dopamine was used as the surface modifier. The linewidth
decreased to DH_pp = 10 Gauss upon using deuterated dopamine
| (ring D₃ or 2,2 D₂). As the signal was dependent on the nature
of the surface modifier, and photogenerated electrons were found
to be involved in the reduction of Ti, this signal can be assigned to
the reversible trap for photogenerated holes. The fact that the
trapping is reversible suggests that oxidation is not followed by
proton loss forming an allylic radical.

By using time-resolved EPR techniques, on the other hand, one
can observe the formation of the initial radical intermediates as well
as monitor their spin dynamics in order to gain insight into the
electron transfer dynamics. The initial spin populations in
photogenerated radicals that are precursors to fully charge separated
states were investigated using time-resolved direct detection (TRDD)
EPR. The EPR spectrum that we have obtained 1 microsecond
after the laser excitation at helium temperatures is composed of
two emissive lines (g=2.004 DH_pp = 11 Gauss and g=1.995
DH_pp= 4 Gauss) and one weakly absorptive line (g=1.988).
The overall shift of the spectrum to net emission suggests that
a triplet state may be involved in determining the electron spin
polarization. The EPR spectrum of dopamine-modified 42 Å TiO₂
shows similar properties, except that the linewidth of dopamine
is larger, as observed in the corresponding cw EPR measurements.
We are exploring these results using multifrequency (X, Q, and
D bands) EPR and simulations to characterize the signal. Our
current thinking is that the spectrum is associated with the exciton
triplet state in which holes, localized on the surface modifier, are
strongly interacting with electrons, delocalized in the conduction
band of TiO₂.

Further investigation of the trapping process at longer times using
the TRDD technique was not successful probably due to the
relaxation of the polarization and low sensitivity of the TRDD EPR
technique. However, by using light having flash duration that exceeds
the relaxation time T₁, one can observe larger concentrations of
nonrelaxed populations and use field modulation to enhance the
sensitivity of the EPR. We have obtained spin polarized signals by
using light-modulated - field-modulated (LFM) EPR measurements
at low temperatures. In these EPR spectra each transition was split
into a doublet of absorptive and emissive lines. In the case of
dopamine-modified samples the signal from photogenerated holes
was significantly broadened. These spectra indicate that upon
trapping onto Ti(III) centers, the interaction of electrons localized
on TiO₂ particles and holes localized on the organic modifier
becomes relatively weak, and therefore stabilizes the charge
separated state in the same manner as in the P⁺Q^- system in the
photosynthetic reaction center. [For the movie on spin polarization
in TiO₂ click here. Netscape users will have to download
and install the Showit Plugin at
http://www.corel.com/products/wordperfect/cwps8/disclaimer.htm
before clicking on movie. If you have the patience for the
version with the music, click here. Note: to return to text, press the
escape key.] This interaction is manifest by observation of the
characteristic correlated radical pair electron spin polarization
(CRPP), a signature for natural photosynthetic systems. Existence
of the spin polarization is the first evidence of the weak interaction
of the electrons localized inside nanocrystalline particles and holes
localized on the surface adsorbed species.

Theoretical models have been used in order to simulate the spectra
and determine the parameters of radical pairs. Theoretical analysis
demonstrates that the experimental spectra are satisfactorily fit with
the model for spin-correlated radical pairs. It was shown that a weak
dipole-dipole interaction (D ~4 Gauss) can account for interaction.
Using computer simulations of the spectra, the distance between
trapped electrons localized within the TiO₂ particle and trapped
holes localized on the surface modifier was found to be R_e-h ~19 Å,
in agreement with the particle radius of 21.5 Å. These results indicate
that photogenerated electrons are localized mainly in the center of the
TiO₂ particles. This result is even more striking because the observed
distance between electrons and holes is larger than the exciton radius
in TiO₂ (< 15 Å). Also, simulations show that the correlation of
orientation of the dipole-dipole interaction vector in the magnetic
axes of electrons and holes is preserved.

Future Research

Several time-resolved EPR techniques will be applied in order to
confirm our time-resolved and light-modulation results. In addition,
pulsed EPR methods will be employed. The electron spin echo
envelope modulation will be used to characterize the interaction
of the paramagnetic charge carriers with their local environments
and determine zero field splitting D more accurately. High-field
EPR (140 GHz, D-band) in conjunction with isotopic labeling will
give further insight into charge separation processes in small particle
semiconductors (triplet vs. interacting radicals).

The charge separation distances in surface-modified metal oxide
colloids will be further increased by binding the amino group of
dopamine to terminus carboxylate-modified oligonucleotides of
defined sequence via the intermediate N-hydroxy-succinimide
ester. The oligonucleotides will be chosen to have hole trapping
sites at different distances from the particle surface. Transient
absorption studies will be performed in order to monitor the kinetics
of primary photochemistry of diverse oligonucleotide-modified
nanocrystallites. We will continue to employ magnetic resonance
techniques in order to study binding and the charge separation
mechanisms in oligonucleotide-modified TiO₂ nanocrystalline
particles. Small angle X-ray scattering techniques will be employed
to obtain the structure of two hybridized oligonucleotide-modified
nanoparticles with varying oligonucleotide length.

Binding and sequential electron transfer mechanisms will be
investigated by examining the changes in local symmetry, hyperfine
interactions, and spin lattice coupling along the pathway of charge
carriers in oligonucleotide-modified nanoparticles. The correlation
between the charge separation distances and the efficiency of
photochemical conversion, as well as the influence of the
hydrophobic character of particle surfaces on their redox and
dynamic properties, will be investigated in a series of surface
modifiers with different chain lengths.

Organization of particles in supramolecular assemblies can affect
their electronic properties. Self-organization of 45 Å TiO₂ particles
into the assemblies with a preferential conduction axis can result in
a system capable of anisotropic conductivity by interparticle electron
transfer. We have previously observed, using small angle neutron
scattering and transmission electron microscopy, the assemblage
(into chains) of TiO₂ nanocrystalline particles upon surface
modification. The existence of interparticle electron transfer, as well
as the correlation between the structure and charge separation
distances, will be probed following the change in the transient
absorption and magnetization after charge separation in assemblies
of different length. The interparticle electron transfer efficiency will
be examined with different chain lengths of surface modifiers and
different interparticle distances. Surface-modified TiO₂ particles
will also be incorporated into self-assembled matrices.

Contact: T. Rajh

Return to Hierarchial Photosynthetic System

Return to Photosynthesis

Glassblowing

Interfacial Processes

Radiation and Photochemistry

Photosynthesis
Biological Materials Growth Facility

Cluster Studies

Chemical Dynamics

Atomic Physics

Nanophotonics

Heavy Elements

Coordination Chemistry

f-Electron Interactions

Actinide Facility

Computational Materials and Electrochemical Processes